It is known that the release of combustion products, such as the waste gases of a power plant boiler into the atmosphere, may result in environmental pollution to the detriment of the surrounding populace and in violation of regulations and statutes and thus systems have been developed to scrub or wash these gases and to remove sulfur therefrom.
The sulfur present in such cases may be in the form of sulfur dioxide and even sulfur trioxide, or in the form of hydrogen sulfide and even carbon disulfide or like products which are generated to various extents depending on the parameter of the production process.
One approach with the removal of sulfur compounds from such exhaust gases is to scrub them with an aqueous solution into which sulfur-binding compounds can be introduced. The scrubbing is generally effected in a scrubbing tower or column in which the gases pass countercurrent to the scrubbing solution which can be introduced in finely divided form into an upper portion of the column. The lower portion or the sump collects the scrubbing liquid and serving as a site at which solids may precipitate. These solids may include dust scrubbed from the gas and solids formed in the desulfurizing reaction. The decanted scrubbing liquid may be recycled from the sump to sprayers at the upper portion of the column.
Basically, therefore, the scrubber or the washing colum which forms part of a desulfurizing apparatus for the waste or flue gases from the power plant boiler combustion chamber will have a flue gas inlet, a flue gas outlet spaced apart along the height of the column from the inlet, means for introducing the scrubbing liquid between the inlet and the outlet, a sump at the lower part of the column, and means for introducing additives into at least the sump for reaction with the contents thereof.
The additives may be supplied at a higher level for reaction with the gas or in the liquid before these solids in their original or in a dissolved form reach the sump. Since the sulfur binding reaction may be increased by the introduction of oxygen into the system, it has been proposed to provide the sump with oxygen feed pipes.
In general, therefore, the portion of a column above the sump operates primarily or exclusively in an absorption mode, i.e. components which are removed from the gas are taken up by the liquid by solubilization or other absorption or incorporation into the liquid phase.
In general, also, the sulfur is bound in the sump in the form of calcium sulfite and calcium sulfate which can be withdrawn from the sump as sludge, slurry or the like.
The scrubbing liquid generally consists of water to which lime, hydrated lime, slaked lime, limestone, dolomite, chalk or the like has been added.
The decanted scrubbing liquid, containing soluble calcium compounds, can then be recycled to absorb additional quantities of sulfur.
It is not necessary to use pure oxygen in such systems and indeed air or oxygen-enriched air is more common. Nevertheless, when reference is made to the introduction of oxygen in the description of the invention it is intended thereby to include these latter oxygen carriers as well as pure or substantially pure oxygen.
In German patent document DE-OS No. 29 28 061, for example, a scrubbing arrangement of the aforedescribed type has been schematically illustrated without any of the particulars of the geometry whereby the oxygen is supplied.
It has also been pointed out/in VGB Kraftwerkstechnik 1981, pages 938-944, where conventional scrubbers for desulfurization purposes are described that the mixtures of calcium sulfate and calcium sulfite which are recovered are proportionally high in calcium sulfite and relatively low in calcium sulfate so that for conversion to gypsum and for use in the building industries, additional steps are required to transform the recovered materials into materials which are more suitable.